Re: [Qemu-devel] [PATCH 07/15] dataplane: use object pool to speed up allocation for virtio blk request

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Aug 01, 2014 at 03:42:05PM +0800, Ming Lei wrote:
> On Thu, Jul 31, 2014 at 5:18 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > Il 31/07/2014 05:22, Ming Lei ha scritto:
> >>> >
> >>> > The problem is that g_slice here is not using the slab-style allocator
> >>> > because the object is larger than roughly 500 bytes.  One solution would
> >>> > be to make virtqueue_pop/vring_pop allocate a VirtQueueElement of the
> >>> > right size (and virtqueue_push/vring_push free it), as mentioned in the
> >>> > review of patch 8.
> >> Unluckily both iovec and addr array can't be fitted into 500 bytes, :-(
> >> Not mention all users of VirtQueueElement need to be changed too,
> >> I hate to make that work involved in this patchset, :-)
> >
> > Well, the point of dataplane was not just to get maximum iops.  It was
> > also to provide guidance in the work necessary to improve the code and
> > get maximum iops without special-casing everything.  This can be a lot
> > of work indeed.
> >
> >>> >
> >>> > However, I now remembered that VirtQueueElement is a mess because it's
> >>> > serialized directly into the migration state. :(  So you basically
> >>> > cannot change it without mucking with migration.  Please leave out patch
> >>> > 8 for now.
> >> save_device code serializes elem in this way:
> >>
> >>  qemu_put_buffer(f, (unsigned char *)&req->elem,
> >>                         sizeof(VirtQueueElement));
> >>
> >> so I am wondering why this patch may break migration.
> >
> > Because you change the on-wire format and break migration from 2.1 to
> > 2.2.  Sorry, I wasn't clear enough.
> 
> That is really a mess, but in future we still may convert VirtQueueElement
> into a smart one, and keep the original structure only for save/load, but
> a conversion between the two structures is required in save/load.

This is a good idea.  The VirtQueueElement structure isn't complicated,
it's just big.  Just use the current layout as the serialization format.

Stefan

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